Catheter pump

ABSTRACT

A catheter pump is provided that includes a rotatable impeller and an elongate cannula. The elongate cannula has a mesh that has a plurality of circumferential members disposed about the impeller. The elongate cannula has a plurality of axial connector extending between a proximal side of a distal circumferential member and a distal side of a proximal circumferential member. The circumferential members are radially self-expandable. The cannula is configured to minimize fracture within at least in the distal zone of the mesh as the elongated cannula moves into a sheathing device.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication, including U.S. patent application Ser. No. 13/801,528 (nowissued as U.S. Pat. No. 9,358,329), filed Mar. 13, 2013, entitledCatheter Pump, and U.S. Application No. 61/667,903 filed Jul. 3, 2012,entitled Catheter Pump, are hereby incorporated by reference under 37CFR 1.57.

BACKGROUND OF THE INVENTION

Field of the Invention

This application is directed to a catheter pump for mechanicalcirculatory support of a heart.

Description of the Related Art

Heart disease is a major health problem that has high mortality rate.After a heart attack, only a small number of patients can be treatedwith medicines or other non-invasive treatment. However, a significantnumber of patients can recover from a heart attack or cardiogenic shockif provided with mechanical circulatory support.

In a conventional approach, a blood pump having a fixed cross-section issurgically inserted between the left ventricle and the aortic arch toassist the pumping function of the heart. Other known applicationsinvolve providing for pumping venous blood from the right ventricle tothe pulmonary artery for support of the right side of the heart. Theobject of the surgically inserted pump is to reduce the load on theheart muscle for a period of time, to stabilize the patient prior toheart transplant or for continuing support. Surgical insertion, however,can cause additional serious stresses in heart failure patients.

Percutaneous insertion of a left ventricular assist device (“LVAD”), aright ventricular assist device (“RVAD”) or in some cases a system forboth sides of the heart (sometimes called biVAD) therefore is desired.Conventional fixed cross-section ventricular assist devices designed toprovide near full heart flow rate are too large to be advancedpercutaneously, e.g., through the femoral artery.

There is a continuing need for improved cannula that provide sufficientexpansion force and a stable expanded shape while still allowing forreliable and easy collapse to a delivery size. In other words, thecannula should have sufficient force to expand, but also be collapsibleunder significant sheathing force while avoiding a risk of damaging thecannula during re-sheathing. There is a continuing need for improvedcannula that can be expanded and collapsed, in some cases over manycycles, without risking breakage of the struts and connectors formingthe mesh. Broken struts pose a risk of complicating patient treatmentand/or compromising the performance of the device.

SUMMARY OF THE INVENTION

There is an urgent need for a pumping device that can be insertedpercutaneously and also provide full cardiac rate flows of the left,right, or both the left and right sides of the heart when called for.

In one embodiment, an apparatus is provided for inducing motion of afluid relative to the apparatus. The apparatus can be a catheter pump,as discussed below. The apparatus can include a rotatable impeller andan elongate cannula. The cannula has a plurality of circumferentialmembers and a plurality of circumferential connectors. Thecircumferential members are disposed about a space, e.g., a volumeincluding at least the impeller zone. One or more of, e.g., each of, thecircumferential members can have an undulating configuration including aplurality of apices connected by elongate struts. The circumferentialconnectors can be disposed between alternating struts of adjacentcircumferential members. A plurality of axial connectors is disposedbetween a proximal side of a proximal apex and a distal side of anadjacent circumferential member in the impeller zone of cannula.

In some embodiments, the cannnula is differentiated along its length tohave varying stiffness. The elongate cannula has an impeller zonedisposed about the impeller and a distal zone disposed distal of theimpeller zone. The distal zone can be made more flexible by reducing thenumber of connectors disposed therein. For example, the impeller zonecan have alternating elongate struts that are connected bycircumferential connectors and the distal zone can have alternatingelongate struts that are free of such connections.

In one variation, each of the connectors of the plurality has a distalend coupled with a proximal side of a proximal apex of a firstcircumferential member, a proximal end coupled with a distal face of asecond circumferential member disposed adjacent to and proximal of thefirst circumferential member. Each of the connectors has an arcuatesection disposed between the proximal and distal ends. In thisvariation, the arcuate section comprises a single convex portiondisposed between the proximal and distal ends.

A first plurality of axial connectors is disposed between a proximalside of a proximal apex and a distal side of an adjacent circumferentialmember in the impeller zone of the cannula. A second plurality of axialconnectors is disposed between a proximal side of a proximal apex and adistal side of an adjacent circumferential member in the distal zone ofthe cannula.

In another embodiment, an apparatus for pumping blood includes arotatable impeller, an elongate cannula, and a sheath configured to bepositioned over the elongate cannula. The elongate cannula has aplurality of circumferential members disposed about the impeller. One ormore of, e.g., each of, the circumferential members can have anundulating configuration. The undulating configuration can include aplurality of proximal and distal apices, with proximal apices connectedto distal apices by an elongate strut. The sheath is configured to bepositioned over the elongate cannula to actuate the cannula from anexpanded configuration to a collapsed configuration. The elongatecannula is configured to deflect radially inwardly in an area around theproximal apices before the apices move into the sheath.

In some configurations, the elongate cannula has an impeller zonedisposed about the impeller and a distal zone disposed distal of theimpeller zone. For example, the impeller can extend in about one-half orless of the length of the cannula. The distal zone may be more easilycompressed by the sheath because the impeller is not present in thatarea. As a result, the cannula can have a different configuration in thedistal zone.

In another configuration, a catheter pump is provided. A catheter pumpis an example of an apparatus for inducing motion of a fluid relative tothe apparatus. The catheter pump includes a rotatable impeller and anelongate cannula having a mesh comprising a plurality of circumferentialmembers disposed about the impeller. The mesh also has a plurality ofaxial connectors extending between a proximal side of a distalcircumferential member and a distal side of a proximal circumferentialmember. The circumferential members are radially self-expandable. Asheath is configured to be positioned over the elongated cannula toactuate the cannula from an expanded configuration to a collapsedconfiguration. The cannula is configured to minimize a risk of fracturewithin the mesh, e.g., of the axial connectors, as the elongated cannulamoves into the sheath.

In a further embodiment, an apparatus for inducing motion of a fluidrelative to the apparatus is provided. The apparatus includes arotatable impeller and an elongate cannula. The elongate cannula isdefines a blood flow channel in which the impeller is disposed. Thecannula has an expandable structure comprising that has a plurality ofcircumferential members, a plurality of circumferential connectors, anda plurality of axial connectors. The circumferential members aredisposed about the blood flow channel. The circumferential members havean undulating configuration including a plurality of apices connected byelongate struts. The circumferential connectors are disposed betweenalternating struts of adjacent circumferential members. The axialconnectors have a distal end coupled with a proximal side of a proximalapex of a first circumferential member, a proximal end coupled with adistal face (e.g., an edge) of a second circumferential member disposedadjacent to and proximal of the first circumferential member. The axialconnectors have an arcuate section disposed between the proximal anddistal ends. The arcuate section comprises a single convex portiondisposed between the proximal and distal ends.

In another embodiment, an apparatus for pumping blood is provided thatincludes a rotatable impeller, an elongate cannula, and an axial member.The elongate cannula has a plurality of circumferential members disposedabout the impeller. One or more of the circumferential members has anundulating configuration including a plurality of proximal and distalapices. Each proximal apex is connected to a distal apex by an elongatestrut. The axial member has a distal end coupled with a proximal apex ofa first circumferential member a proximal end coupled with a secondcircumferential member disposed proximal of the first circumferentialmember. The axial member has a length that is less than the distancebetween the proximal apex to which the distal end of the axial member isconnected and a proximal apex of the second circumferential member thatis axially aligned with the proximal apex to which the distal end of theaxial member is connected.

In another embodiment, a catheter pump is provided that includes arotatable impeller and an elongate cannula. The elongate cannula has amesh that has a plurality of circumferential members disposed about theimpeller. The elongate cannula has a plurality of axial connectorextending between a proximal side of a distal circumferential member anda distal side of a proximal circumferential member. The circumferentialmembers are radially self-expandable. The cannula is configured tominimize fracture within at least in the distal zone of the mesh as theelongated cannula moves into a sheathing device.

In another embodiment, a cannula for conveying blood is provided thatincludes an in-situ expandable and collapsible mesh structure and apolymeric enclosure. The in-situ expandable and collapsible meshstructure has a plurality of undulating circumferential memberssurrounding a lumen. Each circumferential member has proximal and distalvertices. The polymeric enclosure is disposed about the mesh structureto enclose the lumen along a length between an inlet and an outlet. Thenumber of proximal vertices in an area defined between a proximalcircumference intersecting the proximal vertex of a circumferentialmember and a distal circumference intersecting the distal vertex of thesame circumferential member adjacent to the proximal vertex is at leasttwo.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the subject matter of this applicationand the various advantages thereof can be realized by reference to thefollowing detailed description, in which reference is made to theaccompanying drawings in which:

FIG. 1 illustrates one embodiment of a catheter pump configured forpercutaneous application and operation;

FIG. 2 is a plan view of one embodiment of a catheter assembly adaptedto be used with the catheter pump of FIG. 1;

FIG. 3 show a distal portion of the catheter assembly of similar to thatof FIG. 2 in position within the anatomy;

FIG. 4 illustrates a wall pattern of a mesh structure in a flatconfiguration, where the mesh structure is configured to provideenhanced flexibility in a distal zone;

FIG. 4A is a detail view of a distal portion of the wall pattern of FIG.4;

FIG. 5 shows a formed mesh structure for a cannula having a wall patternsimilar to that of FIG. 4;

FIG. 5A is a detail view of the distal portion of the formed meshstructure of FIG. 5;

FIG. 6 illustrates another wall pattern of a mesh structure in a flatconfiguration, where the mesh structure is configured to minimizefracture risk;

FIG. 6A is a detail view of a distal portion of the wall pattern of FIG.6;

FIG. 7 shows a cannula including a formed mesh structure having the wallpattern similar to that of FIG. 6 covered with a film layer;

FIG. 7A is a detail view of a distal portion of the cannula of FIG. 7;

FIG. 8 illustrates another wall pattern of a mesh structure in a flatconfiguration, where the mesh structure is configured to provideenhanced flexibility in a distal zone, while minimizing fracture risk;

FIG. 9 shows a formed mesh structure for a cannula having the wallpattern of FIG. 8;

FIG. 10 illustrates another wall pattern of a mesh structure in a flatconfiguration, where the mesh structure is configured to provideenhanced flexibility in a distal zone, while minimizing fracture risk;

FIG. 11 shows a formed mesh structure for a cannula having the wallpattern of FIG. 10;

FIG. 12 is a detail view of a distal portion of the wall pattern ofFIGS. 8 and 10;

FIG. 13 is a detail view of a first variation of the distal portion ofthe formed mesh structure of FIG. 12;

FIG. 14 is a detail view of a second variation of the distal portion ofthe formed mesh structure of FIG. 12;

FIG. 14A-1 is a detail view of a proximal portion of another variationof a wall pattern that is stable and minimizes fracture;

FIG. 14A-2 is a detail view of a central portion of the wall pattern forwhich the proximal portion is shown in FIG. 14A-1

FIG. 14A-3 is a detail view of a distal portion of the wall pattern forwhich the proximal portion is shown in FIG. 14A-1

FIG. 15 illustrates another wall pattern of a mesh structure in a flatconfiguration, where the mesh structure is configured to provide moreports for blood to flow into or out of a cannula formed with thispattern;

FIG. 15A is a detail view of a proximal portion of the pattern of FIG.15;

FIG. 16 is an expanded view of the pattern of FIGS. 15-15A; and

FIG. 17 is a graph of the sheathing force for collapsing a cannula andan impeller for an example wall pattern.

More detailed descriptions of various embodiments of components forheart pumps useful to treat patients experiencing cardiac stress,including acute heart failure, are set forth below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This application is directed to apparatuses for inducing motion of afluid relative to the apparatus. The apparatus can be a catheter pump,e.g., a percutaneous heart pump.

FIGS. 1 and 2 illustrate various features of a catheter pump 10. Thecatheter pump 10 can provide high performance including flow ratessimilar to full cardiac output. The pump 10 includes a motor driven by acontroller 22. The controller 22 directs the operation of the motor 14and an infusion system 26 that supplies a flow of infusate in the pump10. A catheter system 80 that can be coupled with the motor 14 houses animpeller within a distal portion thereof. In various embodiments, theimpeller is rotated remotely by the motor 14 when the pump 10 isoperating. For example, the motor 14 can be disposed outside thepatient. In some embodiments, the motor 14 is separate from thecontroller 22, e.g., to be placed closer to the patient. In otherembodiments, the motor 14 is part of the controller 22. In still otherembodiments, the motor is miniaturized to be insertable into thepatient. Such embodiments allow the drive shaft to be much shorter,e.g., shorter than the distance from the aortic valve to the aortic arch(about 5 cm or less). Some examples of miniaturized motors catheterpumps and related components and methods are discussed in U.S. Pat. No.5,964,694; U.S. Pat. No. 6,007,478; U.S. Pat. No. 6,178,922; and U.S.Pat. No. 6,176,848, all of which are hereby incorporated by referenceherein in their entirety for all purposes.

FIG. 3 illustrates one use of the catheter pump 10. A distal portion ofthe pump 10 is placed in the left ventricle LV of the heart to pumpblood from the LV into the aorta. The pump 10 can be used in this way totreat patients with a wide range of conditions, including cardiogenicshock, myocardial infarction, and acutely decompensated heart failure,and also to support a patient during a procedure such as percutaneouscoronary intervention. One convenient manner of placement of the distalportion of the pump 10 in the heart is by percutaneous access anddelivery using the Seldinger technique or other methods familiar tocardiologists. These approaches enable the pump 10 to be used inemergency medicine, a catheter lab and in other non-surgical settings.Modifications can also enable the pump 10 to support the right side ofthe heart. Example modifications that could be used for right sidesupport include providing delivery features and/or shaping a distalportion that is to be placed through at least one heart valve from thevenous side, such as is discussed in U.S. Pat. No. 6,544,216; U.S. Pat.No. 7,070,555; and US 2012-0203056A1 (now issued as U.S. Pat. No.8,992,406), all of which are hereby incorporated by reference herein intheir entirety for all purposes.

FIG. 2 shows features that facilitate small blood vessel percutaneousdelivery and high performance up to and in some cases exceeding normalcardiac output in all phases of the cardiac cycle. In particular, thecatheter system 80 includes a catheter body 84 and a sheath assembly 88.An impeller assembly 92 is coupled with the distal end of the catheterbody 84. The impeller assembly 92 is expandable and collapsible. In thecollapsed state, the distal end of the catheter system 80 can beadvanced to the heart. In the expanded state the impeller assembly 92 isable to pump blood at relatively high flow rates. FIGS. 2 and 3illustrate the expanded state. The collapsed state can be provided byadvancing a distal end 94 of an elongate body 96 distally over theimpeller assembly 92 to cause the impeller assembly 92 to collapse. Thisprovides an outer profile throughout the catheter assembly 80 that is ofsmall diameter, for example 12.5 French as discussed further below.

Embodiments of the catheter pumps of this application can be configuredwith expandable structures to enhance performance. For example, animpeller for moving blood axially can be provided. The impeller can bepositioned in an expandable cannula. When so positioned, the expandablecannula provides dual function of enclosing a blood flow lumen throughwhich the impeller can act and also housing the impeller. In that sense,the cannula is also an expandable housing. The expandable cannula andimpeller provide a flow rate in the pump that is much larger than wouldbe possible using percutaneous access techniques were these componentsnot capable of expanding. However, it may be possible to reduce flowresistance by increasing the size of a blood-flow cannula even with afixed diameter impeller. Also, it may be possible to sequentiallycollapse the impeller, e.g., by withdrawing the impeller into a rigidring or tubular segment prior to collapsing the impeller. These variantalso benefit from many of the embodiments herein and are within thescope of this application even though the impeller may not be housed inthe cannula at all times or at all.

While these configurations provide excellent flow rates, a challengearises in collapsing the expanded structures prior to removal from thepatient. The collapsing of the impeller assembly 92 is this manner isnot straight-forward. In various embodiments, a mesh is used to supportthe expandable portion of a blood flow conduit in the impeller assembly92. The expandable portion can include a self-expanding structure thatexpands when undulating generally ring-shaped members release storedstrain energy arising from circumferential compression. Compression ofsuch a structure involves transforming axial relative movement of thesheath assembly 88 over the catheter body 84 into a circumferentialcompression force. There is a chance that such movement will cause thedistal end to become lodged between adjacent undulating members. Suchproblems with compression can be more likely to occur when theundulating members are spaced apart by an axial distance that is greaterthan the wall thickness of the distal end of the sheath assembly 88.While the undulating members could be moved much closer together, suchan approach could make the expandable structure too stiff therebyinhibiting collapse of the expandable portion. Some embodiments hereinare configured to reduce this risk, while retaining sufficientflexibility. Some embodiments are configured to prevent connectorsbetween adjacent rings from being deformed around the distal end of thesheath assembly 88. Various aspects of the expandable cannula and/ormesh in accordance with the invention achieve a careful balance ofexpansion force, collapsing force, and structural strength. FIGS. 4-16illustrate various embodiments of mesh structures that can beincorporated into the expandable cannula to provide advantageousperformance in use.

Additional details of the structures disclosed in these figures, andvarious modified embodiments thereof, and method related to the same arediscussed in U.S. application Ser. No. 13/343,617 (now issued as U.S.Pat. No. 8,485,961), filed Jan. 4, 2012, which is hereby incorporated byreference herein in its entirety. In addition, this applicationincorporates by reference in its entirety and for all purposes thesubject matter disclosed in each of the following concurrently filedapplications: application Ser. No. 13/802,556 (now issued as U.S. Pat.No. 9,446,179), entitled “DISTAL BEARING SUPPORT,” filed on the samedate as this application; Application No. 61/780,656, entitled “FLUIDHANDLING SYSTEM,” filed on the same date as this application;application Ser. No. 13/801,833 (now published as U.S. PatentApplication Publication No. 2013/0303969), entitled “SHEATH SYSTEM FORCATHETER PUMP,” filed on the same date as this application; applicationSer. No. 13/802,570 (now issued as U.S. Pat. No. 8,721,517), entitled“IMPELLER FOR CATHETER PUMP,” filed on the same date as thisapplication; and application Ser. No. 13/802,468 (now issued as U.S.Pat. No. 9,421,311), entitled “MOTOR ASSEMBLY FOR CATHETER PUMP,” filedon the same date as this application.

FIG. 4 shows a flat wall pattern 200 of a mesh structure 204 that isconfigured to provide enhanced flexibility in a distal zone 208. Thedistal zone 208 is disposed distally of an impeller zone 212, which is aportion of the mesh structure 204 that is disposed around an impeller inthe catheter assembly of FIG. 2. The impeller can be part of theimpeller assembly 116, as set forth in more detail in the U.S.application Ser. No. 13/343,617 (now issued as U.S. Pat. No. 8,485,961),incorporated by reference above. In some embodiments, the distal zone208 and the impeller zone 212 are distinct from each other, for examplehaving separate structure or performance characteristics. In otherembodiments, the distal and impeller zones 208, 212 are general regionsof an otherwise continuous structure. In some embodiments, the distaland impeller zones overlap. The wall pattern 200 also includes distaland proximal end connection structures 216A, 216B, which are discussedin detail in the '617 application incorporated by reference above, andalso in U.S. application Ser. No. 13/343,618 (now issued as U.S. Pat.No. 8,597,170), which is hereby incorporated by reference herein in itsentirety.

The wall pattern 200 illustrates a plurality of circumferential members224 and a plurality of circumferential connectors 228. In the flat viewof FIGS. 4 and 4A, the circumferential members 224 can be seen to extendtransversely to a longitudinal axis of the pattern 200. In the formedview of FIGS. 5 and 5A, these structures can be seen to extend about thecircumference of the formed mesh structure 204. As discussed below,e.g., in connection with FIGS. 7 and 7A, a cannula is formed byenclosing the circumferential members 224 with a polymer material, e.g.,a film, to create a flow channel open on the ends but otherwise sealedto maximize axial flow through the cannula and the pump. Thus, thecircumferential members 224 and later the cannula are disposed about aspace, e.g., a volume including at least the impeller zone 212.

In various embodiments, the polymer material is a coating disposed aboutthe cannula mesh structure 204. Suitable materials for the polymercoating include, but are not limited to a biocompatible polymer, adrug-eluting polymer, and a functionalized polymer such as athromboresistant material. In various embodiments, the polymer materialis Hapflex™ or Thoralon™. In the exemplary structure, the polymermaterial fills the voids in the mesh structure. The polymer materialalso coats the inner and outer walls such that the mesh structure doesnot come into contact with blood and tissue. In various embodiments, thepolymer material is a thin coating. In various embodiments, the polymercoating has a maximum thickness of less than 10 microns, less than 9microns, less than 8 microns, less than 7 microns, less than 6 microns,less than 5 microns, less than 4 microns, less than 3 microns, less than2 microns, or less than 1 micron. In various embodiments, the polymercoating is formed of a plurality of layers. In various embodiments, thepolymer coating is configured to reinforce the mesh structure. Thepolymer material may be applied by dip coating, molding, spinning on amandrel, or other techniques. One will appreciate from the descriptionherein that the polymer coating may be configured and applied in variousother manners. Further details of suitable materials are set forth inU.S. Pat. Nos. 4,675,361 and 6,939,377, which are incorporated byreference herein in their entireties and for all purposes.

The circumferential members 224 preferably are configured to beself-expandable, also described as self-expanding herein. FIG. 4 showsthat in one embodiment one or more of the circumferential members 224can have an undulating configuration. As can be seen in many of thefigures, and in FIG. 4A specifically, circumferential members can havean alternating structure, e.g., a plurality proximal turns and distalturns connected by struts. The struts can be straight members that eachhave a proximal ends connected to a proximal turn and a distal endconnected to a distal turn. The distal turns can be peaks and theproximal turns can be valleys, e.g., if the cannula is held with thedistal end up. The distal turns can be crests and the proximal turns canbe troughs, e.g., if the cannula is held with the distal end up. In somecases, the circumferential members have a generally serpentineconfiguration, or can be sinusoidal in nature disposed on both sides ofa transverse plane. The circumferential members 224 can include aplurality of distal and proximal apices 232A, 232B connected by elongatestruts 236. As discussed further below, the density of thecircumferential members 224 can be varied to modify the performance orthe cannula.

The circumferential connectors 228 can be disposed between alternatingstruts 236 of adjacent circumferential members 224 in at least one ofthe distal and impeller zones 208, 212. For example, the connectors 228(or other connectors discussed herein) can join a node on onecircumferential member to a node on an adjacent circumferential member.In the case of the connectors 228, the nodes are offset from the peaksand valleys. At least one of the nodes can be disposed on a strut thatextends between adjacent nodes. In some cases, connectors are disposedbetween the crests and trough and can be disposed between a crest the atransverse mid-point of a sinusoidal circumferential member. In somepatterns, the width of nodes are greater in the impeller zone thandistal thereof. FIG. 4 shows that circumferential connector 228 can beprovided between opposing sides of adjacent struts 236 of at least twoadjacent circumferential members 224 in the impeller zone. In oneembodiment, alternating elongate struts 236 are connected to an adjacentelongate strut of an adjacent circumferential member 224. In oneembodiment, alternating elongate struts 236 are not connected toadjacent elongate struts by circumferential connectors 228. The elongatestruts 236 that are not connected by circumferential connectors 228 areable to expand to a greater degree, providing for asymmetrical expansionabout an apex in some embodiments. In this context, asymmetrical canrefer to unequal movement upon expansion of the elongate struts 236 awayfrom a central axis extending through an unexpanded apex. The centralaxis can be an axis intersecting an apex and being located equaldistance from inner edges of unexpanded adjacent elongate struts 236.

FIG. 4 shows that while circumferential connectors 228 are provided inthe impeller zone 212, the connectors 228 can be omitted in the distalzone 208. Such an arrangement provides enhanced rigidity of the impellerzone 212 compared to the distal zone 208. FIGS. 6 and 14A-1 to 14A-3 areother embodiments in which circumferential connectors 228 are providethroughout a distal zone as well as in an impeller zone. More generally,the circumferential connectors 228 can be provided between opposingsides of alternating adjacent elongate struts of less than all of thecircumferential members in the distal zone 208, while still providingbenefits as discussed below. For example, a substantial portion such asone-half or more of the struts 236 can be connected by circumferentialconnectors 228 in the distal portion in one embodiment. In someembodiments, the density of connectors 228 in the distal zone 208 can beabout one-half or less that in the impeller zone 212.

In various embodiments, there can be different groups of circumferentialconnectors 228. For example, in FIG. 4 two groups of circumferentialconnectors 228A, 228B can be provided. A first plurality connectors 228Acan be provided about the impeller zone 212 in which the connectors havea length along the struts 236 that the connectors 228A join that isgreater than the separation between adjacent circumferential members,e.g. between the struts 236 that they join. The length of the connectors228A along the struts 236 that they join is greater than the separationbetween adjacent circumferential members in the unexpanded state in someembodiments. A second plurality connectors 228B can be provided betweenthe impeller an distal zones 212, 208 in which the connectors have alength along the struts 236 that they join that is less than the lengthof the connectors 228A. For example, the length of each of theconnectors 228B along the struts 236 to which they are coupled can beabout one-half that of the first connectors 228A. By providing longerconnectors 228A, enhanced stiffness can be provided in the impeller zone212. This can aid in collapsing the cannula, as discussed below. Longerconnectors 228A also contributes to dimensional stability of theimpeller zone 212, e.g., to minimize variance of a gap between a tip ofan impeller and an inner surface of the cannula in the impeller zone212.

FIG. 4A shows details of a portion of the distal zone 208. For example,a plurality of axial connectors 252 can be provided between proximalside of a proximal apex and a distal side of an adjacent circumferentialmember in the distal zone of the cannula. The same connectors can beprovided in the impeller zone 212. FIG. 4 shows that in some embodimentsa modified axial connector 252A can be provided in the impeller zone212. The connectors 252A have a first end that forms an apicalconnection with a proximally oriented apex 232B and a second end. Thesecond end is coupled in between adjacent apices of a circumferentialmember, e.g., along a side of an elongate strut 236. The connectors 252Acan be connected to the struts 236 at the same location that the strutconnects to an adjacent strut by way of the connector 228A. Theconnectors 252A are shortened compared to the connectors 252. Forexample, they can extend with fewer undulations along their length,e.g., with a single inflection point between the ends.

In one embodiment, substantially all of the impeller zone 212 hasenhanced rigidity connectors 252A. FIG. 5 shows a transition zone thatcan be provided at one or both of the proximal and distal ends of theimpeller zone 212. For example, a transition zone TZ-B can be providedto facilitate radial transition from the expanded size of the formedmesh to the diameter of a non-expanding portion 254 of the formed meshthat does not expand. A transition zone TZ-A between the impeller anddistal zones can provide for more gradual change in mechanicalcharacteristics to provide for gradual collapse of the cannula, asdiscussed below.

FIG. 5 shows an expanded mesh structure 204 formed of the pattern 200.The mesh structure 204 comprises a plurality of spaced apart helicalzones 260. The helical zones 260 are formed by adjacent struts 236 thathave less or no movement relative to each other during expansion, whereconnectors 228 are provided. Adjacent struts that are connected byconnectors 228 tend to move or expand less than struts that are not soconnected, or do not move or expand at all. FIGS. 5 and 5A shows thatthe helical zones 260 are in the impeller zone 212 and not in the distalzone. The helical zones 260 provide enhanced concentration of materialaround the proximal apices 232B of circumferential members 224 in theimpeller zone 212. Enhanced concentration of material provides enhancedlocal stiffness around the proximal apices 232B, which provides greaterstiffness and protects the proximal apices 232B and connectors disposedthereon from fracture. As discussed more below in connection with FIG.12, in some variants proximal apices in a zone (e.g., a distal zone) areomitted but the helical zones are induced to preserve this protectivestructure around the proximal apices. Such arrangements aid in there-sheathing of a cannula incorporating this structure.

Enhanced concentration of material (e.g., increased struts per unitarea) makes the connection between the apices 232B and adjacent proximalcircumferential member 224 more robust. In particular, larger forces areencountered in the impeller zone 212 during collapsing of, sometimesreferred to as re-sheathing of, the cannula, e.g., after the catheterpump has been used.

FIG. 17 shows a graph of axial force that may be required to be appliedto an outer sheath, such as by the sheath assembly 88, disposed aboutthe catheter body 120 (see FIG. 2) as the sheath assembly is advanceddistally along the cannula. This figure shows that in region A the forceis relatively high when the distal end of the sheath initially engagesthe proximal end of the expanded zone of the cannula. The force alsoincreases in a region B to a relatively high level when the distal endof the outer sheath is advanced to a location over the proximal end ofthe impeller. At each of these locations, the clinicians will noteincreased resistance to advancement of the sheath to collapse theimpeller and/or the cannula and to draw the impeller and/or the cannulainto the outer sheath, sometimes referred to herein as re-sheathing.From this graph, an increased risk of failure of one or more connectionswithin the mesh of the cannula being tested has been discovered in theseregions of local maxima. Surprisingly, however, various cannulae thathave been tested have failed in the lower force region to the right ofregion B. As a result, there is a surprising benefit to be obtained inenhancing the robustness of the cannula in a region distal of theimpeller zone. As discussed herein, the robustness can be enhanced inany one or all of a variety of ways, such as shortening axially orientedconnectors between adjacent circumferential members, increasing materialper unit area in a region around proximally oriented apices, providingan expanded configuration in which struts of the mesh are positionedclose together around a weak point in the mesh structure, and other waysdescribed herein.

FIGS. 4-5A and other embodiments herein illustrate some specific ways ofimproving the reliability of an expandable cannula, which can help tominimize the risk of breakage within the mesh, e.g., breakage of theconnectors 252A. A first technique is making the connectors 252A shorterthan the connectors 252. A second technique involves the enhancedconcentration of material around the proximal apices 232B, discussedabove.

The distal zone 208 is substantially free of the helical zones 260 orother concentration of material in the embodiment of FIG. 4-5A. In thedistal zone 208, the struts 236 expand substantially symmetrically aboutthe apices 232A forming a more uniform expanded mesh in the distal zone208. This arrangement enhances the overall flexibility of the distalzone 208, which can be beneficial. Also, this arrangement makes localzones of the expanded mesh structure 204 substantially uniformlyflexible. A mesh structure 204 with a more flexible distal zone 208, andmore uniform flexibility, can provide a cannula with a reduced risk ofirritating the inner structures of the heart when deployed.

FIG. 6 shows an embodiment, in which the distal zone 208 is modified tominimize a risk of fracturing connectors in the distal zone. Forexample, a first plurality of axial connectors 272 is disposed between aproximal side of a proximal apex 232B and a distal side of an adjacentcircumferential member 224 in the impeller zone 212. FIG. 6A shows asecond plurality of axial connectors 272A is disposed between a proximalside of a proximal apex 232B and a distal side of an adjacentcircumferential member 224 in the distal zone 208 of the cannula. In theillustrated embodiment, connectors 272A are provided between proximalapices 232B and a middle portion of the elongate members 236. The axialconnectors 272A of the second plurality have first and second ends, anda singe curved section therebetween. In contrast, the axial connectors272 of the first plurality have first and second ends, and a pluralityof curved section therebetween. Stated another way, the axial connectors272 have multiple undulations and the connectors 272A have a fewerundulation, e.g., a singe curved section.

Also, the distal zone 208 can be made less susceptible to fracture byproviding circumferential connectors 228 in some embodiments. In theillustrated embodiment, every other elongate struts 236 of acircumferential member 224 in the distal zone 208 is connected to anadjacent elongate struts 236.

FIGS. 7-7A show a cannula 296 incorporating the formed shape of a meshwith some features similar to those of the flat pattern of FIGS. 6 and6A. In this embodiment the helical zones 260 are provided throughout thecannula 296. In the embodiment of FIGS. 7-7A, apex-to-side connectorseach have a plurality of undulations along their length, similar to theconnectors 272. However, providing side-to-side connectors 228throughout the length of the cannula 296 enhances the concentration ofmaterial around the apex-to-side connectors. As such, the load appliedby the advancement of the sheath over the cannula is spread out over agreater area and a smaller load is concentrated in the apex-to-sideconnectors. These are examples of techniques for minimizing the chanceof fracture of connectors similar to the connectors 272, 272A.

FIGS. 8 and 9 illustrate an embodiment that is similar to that of FIGS.6 and 7. FIGS. 8 and 9 show a wall pattern 300 and a mesh structure 304that is configured to provide a good compromise of fracture resistanceand flexibility for interaction with heart tissue. The impeller zone 312has a plurality of circumferential connectors 328 whereas the distalzone 308 is substantially free of circumferential connectors. Thisprovides an impeller zone with spaced apart helical zones, as discussedabove, and a distal zone with substantially symmetrical expansion aboutproximally and/or distally oriented apices thereof. By removing thecircumferential connectors in the distal zone, the distal zone is mademore flexible. Axial connectors 352 are provided throughout the pattern300, but the distal zone 308 is provided with axial connectors 352A thatare less subject to fracture. For example, the connectors 352A can belimited to fewer undulations than in the connectors 352 in the impellerzone 312, as discussed above.

FIGS. 10 and 11 illustrate an embodiment that is similar to that ofFIGS. 6 and 7. FIG. 10 shows a wall pattern 400 that has a higherconcentration of material in the impeller zone 412. Higher concentrationof material can be achieved by more tightly packing the apices of theundulating structure of the circumferential members. For example, anangle can be formed between adjacent elongate struts 436 disposed onopposite sides of each of the apices. The angle can be smaller in theembodiment of FIGS. 10 and 11 compared to that of FIGS. 6 and 7. Also,in order to provide more flexibility compared to the pattern of FIGS. 6and 7, the pattern of FIGS. 10 and 11 omits circumferential connectorsin the distal zone 408.

One technique for minimizing fracture risk in the distal zone 408 is toconfigure the mesh structure 404 to produce helical zones 460 throughoutthe structure, including in the distal zone 408 where there area nocircumferential connectors. This can be achieved by heat setting theexpanded shape in a material that would operate in an elastic range inthis application. For example, nitinol can be configured to becompressed for delivery and heat set to expand to the shape seen in FIG.11. This arrangement may provide good flexibility in the distal portion408 and resistance to fracture of connectors between adjacentcircumferential members 424.

FIGS. 12-14 illustrate an embodiment in which at least a distal zone isconfigured to be resistant to fracture. FIGS. 12 and 13 show that in oneembodiment, short connectors 452 are positioned in the distal zone. Theshort connectors 452 can be similar to those discussed above, e.g.having only a single curve or inflection between ends thereof. In oneconfiguration, the short connectors 452 can have a length that is nomore than about ten times the thickness of the outer sheath used tocollapse the cannula. For example, these connectors can be about 0.035inches long or less. The connectors 452 can be robust in their own rightto permit a symmetrical expanded configuration for the distal zone ofthe wall pattern. Symmetrically expanded apices can provide someadvantages, e.g., providing more uniform flexibility with the meshstructure, as discussed above. In some embodiments another technique canbe used to spread a load applied by an outer sheath to the cannulaincorporating the mesh illustrated in these figures. By spreading theload, the mesh is less subject to fracture.

The pattern arrangements in FIGS. 10-14 also reduce stress and strain onthe connectors by subjecting them to more pulling and less twistingduring expansion and collapse of the mesh structure. Another advantageof these designs is that they tend to deflect the proximally orientedapices radially inward as the sheath approaches the proximally orientedapices so that a distal face of the sheath does not become lodgedbeneath the proximally oriented apices. In the heat set distal portionillustrated in FIG. 14, each connector 452 is disposed distal of aportion of an elongate strut 436. The connectors 452 also can be locatedaxially behind the elongate struts 436. In contrast, in the embodimentof FIG. 13, the connector 452 is disposed distal of the nearestproximally oriented apex. The elongate struts 436 in the FIG. 14embodiment, helps guide the approaching outer sheath over the top of theproximally oriented apices. The elongate struts 436 also locally deflectthe cannula in a zone between the connectors 452 and the outer sheath asthe sheath approaches individual connectors to minimize any tendency ofthe connectors 452 deforming around the distal end of the sheath andlater breaking.

Another approach to easing re-sheathing involves reducing an amount ofopen area in the formed cannula wall pattern around relative stiffproximally oriented structures. For example, the axially orientedundulating connectors in FIG. 5A may be more prone to fracture. At leasttwo factors contribute to this. First, these connectors are relativelylong. Also, they are surrounded on both sides by large areas not spannedby struts of the mesh pattern. These areas are covered with a polymermaterial to enclose a cannula. However upon re-sheathing this polymermaterial can ride over the outer surface of the outer sheath causing theproximal apices to ride over the outside of the sheath. This can lead tobreakage of the axially oriented undulating connectors in FIG. 5A. Thisis input because the axially oriented connectors are relatively thin inat least one cross-sectional dimension and are formed of somewhatductile metal. FIG. 13 shows the connectors 452 shortened to minimizethis effect. FIG. 14 shows an embodiment in which more struts are placedaround at least one side of the short connectors 452 in the helicalzones 460. Additionally, circumferential connectors can be provided inthe distal zone for this purpose, as discussed herein.

FIG. 14A-1 to 14A-3 show a proximal portion 462A, a central portion462B, and a distal portion 462C of a wall pattern 462. Each of theportions 462A, B, C has a plurality of circumferential members 466 in arelatively high metal density structure. The circumferential members 466are close to each other in each of these portions. The members 466 haveundulating configurations, e.g., with peaks and valleys. The peaks andvalleys of neighboring members 466 can be received within each other, asshown in the figures. The proximal portion 462A is configured to enhancestructural integrity of the wall surrounding the impeller. Advantagesfor this arrangement are discussed above, and include minimizingvariation in the gap between the tip of the impeller and the inner wallof the cannula into which the pattern of FIGS. 14A-1 to 14A-3 isincorporated. In the illustrated embodiment, the stiffness of theproximal portion 462A is enhanced by providing a plurality of elongatecircumferential connectors 470. The advantages of this sort of connectorare discussed above, and include minimizing expansion of elongate struts474 which are coupled by the connectors 470.

As shown in FIG. 7 an expanded cannula with connectors 470 will have anexpanded configuration including spaced apart helical spines that arisefrom the minimal to no displacement of the struts 474 that are connectedby the connectors 470. The spines or other configurations including aconnector 470 and a plurality of struts connected thereby advantageouslyprovide areas of enhanced stiffness and/or strength in the wall of acannula having the pattern 462. Such regions can support an outward loadwithout significant deflection. One outward load that can arise inoperation is due to the fluid flowing in the cannula. Although theimpeller is configured to primarily drive the blood axially therotational movement may push blood into the inside wall of the cannula.The spines or other areas of enhanced stiffness can minimize deflectiondue to this load. Another outward load can be applied by a distalbearing structure such as that described in concurrently filedapplication Ser. No. 13/802,556 (now issued as U.S. Pat. No. 9,446,179),entitled “DISTAL BEARING SUPPORT,” filed on the same date as thisapplication, which is hereby incorporated herein by reference in itsentirety.

In addition to the connectors 470, 470A, the pattern 462 includesconnectors 476 disposed between proximal oriented apices (or valleys, ifthe pattern is held distal end up) and distal edge of a circumferentialmember disposed proximally of the apex. The connectors 476 arerelatively slender in order to permit the apices to which they areconnected to flex upon expansion and collapse of the cannula into whichthe wall pattern 462 is incorporated. The proximal portion 462A alsoprovides enhanced concentration of material around the connectors 476 tominimize a chance of fracturing these connectors upon expansion andcollapse of the cannula.

FIG. 14A-1 illustrates a pattern in the proximal portion 462A providinga high metal density cannula. The pattern has an embedded ringstructure, which provides a first ring with a vertex of at least oneadjacent ring within the axial length of the first ring. FIG. 14A-1illustrates embodiments where there are 2 or more adjacent verticeswithin the axial length of the first ring. There are several benefits ofthe embedded design for the pump 10. The embedded design providesadditional radial strength by increasing the number of load bearingrings per length of cannula. The embedded design minimizes theunsupported film area, which decreases the amount the film can flex asthe pressure pulses generated by the impeller pass under it.

Other methods can be used to provide some of these benefits, includemaking the axial length of the rings shorter and increasing the numberof sinusoidal waves within a ring. These features will result in otherchanges to the cannula pattern. Shorter rings tend to increase thestrain the material must undergo in changing from the collapsed toexpanded state. Suitable materials, such as nitinol, may be more proneto permanent deformation or fracture with increasing strain. Increasingthe number of sinusoidal waves within a ring increases the diameter ofthe collapsed cannula, if other relevant factors (such as strut width)remains the same, may reduce the stress and strain to which the strutsare subjected.

Other cannula patterns can be provided to reduce unsupported film areawithout increasing the number of embedded rings. For example, an armfeature could be added between two struts of adjacent rings. As thecannula transforms from the collapsed to expanded form, the armorientation moves from a more axial to more radial orientation (in somecases, forming an “A” shape). More details of these structures are setforth in connection with FIGS. 15-16 below.

In a transition zone between the proximal portion 462A and the centralportion 462B, modified connectors 470A are provided that are muchshorter in a direction parallel to the longitudinal axis of the struts474 than are the connectors 470. The shorter connectors 470A make thecentral portion 462B much more flexible than the proximal portion 462A.Such flexibility can provide less irritation to heart tissue and thanhigher biocompatibility as discussed elsewhere herein. FIG. 14 showsthat the connectors 470A also are provided in the distal portion 462C.

A proximal end of each of the connectors 476 is coupled to a portion ofthe strut 474 that is also connected to the connector 470 or 470A. Thisstructure provides a concentration of material around the more flexibleand elongate connector 476 to minimize the chance of fracture of thisstructure when the cannula is collapsed by a sheath, as discussedherein. Because the sheathing forces are less in the distal portion ofthe cannula corresponding to the distal portion 462C, the concentrationof material in the distal portion 462C around the connectors 476 can beless than in the proximal portion 462A.

Various additional advantageous features are found on the proximal anddistal portions 462A, 472C in various embodiments. For example, when thepattern 462 is formed a sheet-like zone 480 is provided that isadvantageous for mechanically integrating the pattern 462 into variouscatheter bodies in an assembly. A plurality of cantilevered projections482 is disposed in the sheet-like zone 480 and is disposed about thecircumference of the proximal portion 462A when the pattern 462 isformed into a tubular body. The projections 482 can be deflected intomating recesses in the catheter body 84 or another structure of acatheter assembly to provide a resistance to detachment of the pattern462 (and the cannula into which it is incorporated) from the catheterbody or assembly. Such resistance advantageously minimizes inadvertentseparation of the cannula from the catheter body 84 during re-sheathing.

Also, the peak-to-peak distance between the proximal-mostcircumferential member 466 and the circumferential member immediatelydistal thereof is greater than the average peak-to-peak distance ofadjacent circumferential members distal thereof. A consequence of thisis that the connector 470 between the proximal-most circumferentialmember and the adjacent circumferential member is located closer to thepeaks of the proximal most circumferential member. This creates anenlarged space 488 that aids in transitioning the diameter of theexpanded cannula into which the pattern 462 is incorporated from alarger size disposed about the impeller to the diameter of thesheet-like zone 480 when formed into a tubular body.

The distal portion 462C of the wall pattern includes elongate members490 that are for mechanically integrating the pattern into a catheterassembly. The elongate members 490 extend from distal apices of thedistal portion 462C of the pattern 462. The connectors 470A and 476disposed between the distal-most circumferential member 466 are shiftedcloser to the peak of the adjacent circumferential member 466 such thata larger space 492 is provided between the distal-most twocircumferential members 466. The shifting of these connectors provides alarger peak-to-peak distance between the distal-most two circumferentialmembers 466 than is provided between other circumferential members ofthe pattern 462. By increasing this distance, the transition from theenlarged diameter of the expanded cannula into which the pattern isincorporated to the smaller diameter of distally located non-expandablecomponents of a catheter assembly can be facilitated.

FIGS. 15 and 15A illustrate another embodiment of a wall pattern 500that can be combined any features of any of the wall patterns herein.These embodiments illustrate a proximal zone of the pattern that forms atransition zone between a non-expandable proximal portion 554 of acannula and an impeller zone 512. One feature of a cannula formed fromthe wall pattern 500 is the provision of a larger number of flowpassages between inside of the proximal portion of the cannula andoutside of the proximal portion thereof. A first outflow area isprovided in one embodiment adjacent to the proximal end of theexpandable portion of the cannula. In some embodiments, a second outflowarea 520 is provided distal of the first outflow area. The first andsecond outflow areas 518, 520 can take any suitable form. In theillustrated embodiment, the first outflow area is defined between adistal edge or side of a first circumferential member 524A and aproximal edge or side of a second circumferential member 524B.

FIG. 16 shows the expanded configuration of one of a first plurality ofopenings 528A formed in a mesh. Comparing FIGS. 15A and 16 one can seethat the plurality of openings can include four openings 528A formeddefined within the members 524A, 524B and connectors extendingtherebetween. The connectors can take any suitable form, such as thosediscussed above. In the illustrated embodiment, the second outflow areais defined between a distal side or edge of the second circumferentialmember 524B and a proximal side or edge of a third circumferentialmember 524C. More particularly, a plurality of openings 528B in a mesh,e.g. four opening, formed by the pattern 500 are defined within themembers 524B, 524C and connectors extending therebetween. The connectorscan take any suitable form, such as those discussed above.

FIG. 16 shows a cross-hatched zone distal of the openings 528A, 528B.The cross-hatched zone illustrates the area of the mesh structure thatis covered to enclose the space within the mesh structure. ComparingFIG. 16 with FIG. 9, one can appreciate that the openings 528A areinclined with respect to the longitudinal axis of the spaced enclosedtherein and the openings 528B are less inclined and in some cases may bedisposed on a substantially cylindrical surface about the longitudinalaxis of the cannula. In this context, the concept of conforming to acylindrical surface can be measured when the device is expanded but notimplanted or in use. The mesh structure 500 should advantageouslyprovide beneficial flow characteristics compared to an arrangement thatonly has flow openings 528A. For example, by providing the flow openings528B (or other variant of a second outflow area 520), the average flowvelocity into or out of the cannula can be decreased. By decreasing theaverage flow velocity, stress on the blood cells can be reduced. Suchstresses can be due to shear forces across the boundary into or out ofthe cannula. Lower stresses on red blood cells can lessen hemolysis orother harm to the blood.

FIGS. 15-16 also illustrate the use of a circumferential connector 530that is configured to reduce the extent of an unsupported portion of astructure enclosing a lumen within the mesh 500 after the mesh is formedinto a cylinder. The connector has a proximal end 530B coupled with aproximal circumferential member 524B and a distal end 530C coupled witha circumferential member 524C that is located distal of thecircumferential member 524B. The length of the connector 530 is severaltimes the unexpanded separation distance between the adjacent struts ofthe circumferential members 524B, 524C. The length of the connector 530enables the adjacent struts of the circumferential members 524B, 524C tomove away from each other to a much greater extent than permitted by theshort circumferential connectors 470. In the collapsed state, theconnector 530 can be shaped to tightly nestle between thecircumferential members 524B, 524C, for example, having a concaveportion adapted to receive a portion of a crest of the circumferentialmembers 524B. In some embodiments, the connector 530 enables theadjacent struts of the circumferential members 524B, 524C to move awayfrom each other to the same extent as if these struts were not connectedby a circumferential member. See, for example, the struts of thecircumferential members that are not connected by circumferentialconnectors in the expanded cannula shown in FIG. 7A. However, as can beseen in FIG. 16, the presence of the connector 530 greatly reduces theextent of the unsupported area between adjacent struts.

In one embodiment, the connector 530 is connected approximately in themiddle of adjacent struts, e.g., half way between adjacent peaks andvalleys on each circumferential member. This arrangement roughly reducedby 50 percent the unsupported area between these struts. Long slenderstruts may be more prone to shearing upon being collapsed into thesheath. Accordingly, it may be desirable to locate the struts 530 inareas of local minima of a sheathing force curve as discussed below inconnection with FIG. 17. In other embodiments, the connectors 530 arelocated away from areas of local maxima of a sheathing force curve asdiscussed below in connection with FIG. 17.

FIGS. 5, 7, and 17 illustrate further advantageous features of wallpatterns. In particular, as noted above, a local maximum in theforce-distance curve of FIG. 17 is the region A, which corresponds to atransition zone between the non-expandable part of the mesh and theexpandable impeller zone. The test illustrated in FIG. 17 shows thatthis local maximum exceeds a threshold number that is based on clinicianease-of-use. On technique for reducing the level of this local maximumis to provide a shallower angle of the transition zone. In particular,an angle β can be provided between this inclined surface and ahorizontal axis, e.g., an axis parallel to the undeflected longitudinalaxis of the cannula. FIG. 7 shows a smaller angle than that of FIG. 5.Preferably the angle β is within a range of from about 30 to about 40degrees, in some embodiments not more than about 40 degrees. To reducethe force required for re-sheathing, the angle β can be maintained atabout 30 degrees or less. The angle β may be maintained above a valuethat is a function of the trackability of the catheter assembly intowhich the mesh structure is incorporated. If the angle is too low, thelength of the cannula or portions thereof may result in a too stiffcannula to properly track. Another advantage of the shallower anglessuggested by FIGS. 7 and 17 is that the impeller zone in a cannulaincorporating this pattern is expected to be stiffer. This is onetechnique for providing better control of a gap between the impellerblade tip and the cannula wall. This tip gap control can advantageouslyminimize hemolysis and other damage to the blood, as well as any damageto the wall or blades that could be cause by impact therebetween.

The foregoing features of mesh patterns can be combined with otherfeatures for improving tip gap control. Such features can beincorporated into a distal bearing, as discussed in concurrently filedapplication Ser. No. 13/802,556 (now issued as U.S. Pat. No. 9,446,179),entitled “DISTAL BEARING SUPPORT,” filed on the same date as thisapplication; and/or in impeller or impeller blade construction, asdiscussed in application Ser. No. 13/802,570 (now issued as U.S. Pat.No. 8,721,517), entitled “IMPELLER FOR CATHETER PUMP,” filed on the samedate as this application. Both of these applications are incorporated byreference herein in their entirety.

Although the inventions herein have been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent inventions. It is therefore to be understood that numerousmodifications can be made to the illustrative embodiments and that otherarrangements can be devised without departing from the spirit and scopeof the present inventions as defined by the appended claims. Thus, it isintended that the present application cover the modifications andvariations of these embodiments and their equivalents.

What is claimed is:
 1. An apparatus for inducing motion of a fluidrelative to the apparatus, comprising: a rotatable impeller; and anelongate cannula defining a blood flow channel in which the impeller isdisposed, the cannula comprising an expandable structure comprising: aplurality of circumferential members disposed about the blood flowchannel, the circumferential members having an undulating configurationincluding a plurality of apices connected by elongate struts; aplurality of circumferential connectors disposed between alternatingstruts of adjacent circumferential members; and a plurality of axialconnectors comprising a distal end coupled with a proximal side of aproximal apex of a first circumferential member, a proximal end coupledwith a distal face of a second circumferential member disposed adjacentto and proximal of the first circumferential member, and an arcuatesection disposed therebetween, wherein the arcuate section comprises asingle convex portion disposed between the proximal and distal ends. 2.The apparatus of claim 1, wherein the elongate cannula comprises animpeller zone disposed about the impeller and a distal zone disposeddistal of the impeller zone, and wherein the axial connectors comprise afirst plurality and further comprising: a second plurality of axialconnectors is disposed between a proximal side of a proximal apex and adistal side of an adjacent circumferential member in the distal zone ofthe cannula, the axial connectors of the second plurality being moreflexible than the axial connectors of the first plurality.
 3. Theapparatus of claim 1, further comprising an enclosure comprisingpolyurethane disposed about the circumferential members to enclose alumen along a length of the cannula between an inlet and an outlet. 4.The apparatus of claim 1, wherein the elongate cannula comprises animpeller zone disposed about the impeller and a distal zone disposeddistal of the impeller zone, and wherein the plurality ofcircumferential connectors are disposed in the impeller zone and are notdisposed in the distal zone.
 5. The apparatus of claim 1, wherein theplurality of circumferential connectors include a first group ofcircumferential connectors having a first length and a second group ofcircumferential connectors having a second length, the first lengthgreater than the second length.
 6. The apparatus of claim 5, wherein thefirst length is greater than an unexpanded separation distance betweenadjacent circumferential members connected by the first group ofcircumferential connectors.
 7. The apparatus of claim 5, wherein thesecond length is about one half of the first length.
 8. The apparatus ofclaim 1, wherein the plurality of circumferential connectors each haveat least one concave portion adapted to receive a portion of a crest ofan associated circumferential member.
 9. The apparatus of claim 1,wherein the plurality of circumferential connectors each have a lengthlonger than multiple times an unexpanded separation distance betweenadjacent circumferential members connected by the circumferentialconnectors.
 10. The apparatus of claim 1, wherein the axial connectorscomprise a first plurality of axial connectors, the apparatus furthercomprising: a second plurality of axial connectors each having aplurality of curved sections.